This invention relates generally to a system and method for controlling a direct injection spark ignition internal combustion engine.
Direct injection spark ignition (DISI) internal combustion engines may be operated in various modes depending upon the particular objectives to be attained at any particular time with emphasis on power output, fuel economy, and/or low emissions, for example. Operating modes may include a homogeneous mode in which the combustion chambers contain a substantially homogeneous mixture of air and fuel, or a stratified mode in which the combustion chamberers contain stratified layers of different air/fuel mixtures. Stratified mode generally includes strata containing a stoichiometric air/fuel mixture nearer the spark plug with outer strata containing progressively leaner air/fuel mixtures.
Typically, there is a range of air/fuel ratios within which stable combustion can be achieved in the stratified mode, such as between 25:1 and 40:1, and a second range in which stable combustion can be achieved in the homogeneous mode, such as between 12:1 and 20:1. As such, there is typically a significant gap between the leanest air/fuel ratio of the homogeneous mode (20 in this example), and the richest air/fuel ratio of the stratified mode (25 in this example). This gap poses a number of problems in selecting an appropriate operating mode and controlling the engine. For example, best fuel economy is often associated with highest allowable manifold pressure which may dictate an air/fuel ratio which falls within the gap and is therefore not achievable in either mode of operation. As such, the engine controller operates the engine at a richer air/fuel ratio to maintain stable combustion with a resulting lower fuel economy. It also represents a challenge during mode switch when the transient manifold pressure and air charge in-cylinder reach conditions which prevent the engine from producing exactly the demanded torque without violating air-fuel ratio constraints.
In accordance with the present invention, a method and system is provided for controlling an internal combustion engine, such engine being adapted to operate in a selected one of a plurality of operating mode. One of such modes is characterized by stratified combustion and another one of such modes is characterized by a homogeneous combustion. The method and system include determining, from operating conditions of the engine and a demanded torque, a preferred one of the operating modes. A determination is made from such demanded torque whether operation such preferred operating mode is feasible. If such preferred mode is determined to be feasible, the engine transitions to the preferred mode; otherwise, such engine is operated in one of the modes which provides the minimum difference between the torque produced in such one of the modes and the demanded torque.
In one embodiment, the feasibility determining comprises determining maximum torque, xcfx84max,s, producible by such engine in the stratified operating mode and determining minimum torque, xcfx84min,h, producible by such engine in the homogeneous operating mode. The method and system determine that operation of the engine is feasible in the stratified mode if the demanded torque is less than or equal to the determined maximum torque, xcfx84max,s, and determine that operation of the engine is feasible in the homogeneous mode if the demanded torque is greater than or equal to the determined minimum torque, xcfx84min,h.
In one embodiment, maximum torque, xcfx84max,s, is determined as a function of engine operating speed, air charge, burned gas fraction in a cylinder of such engine and a lower limit of the air fuel ratio for stratified operation of such engine.
In one embodiment, minimum torque, xcfx84min,h, is determined as a function of engine operating speed, air charge in a cylinder of such engine, an upper limit of the air fuel ratio for homogeneous operation of such engine, burned gas fraction in the cylinder, and maximum delayed ignition timing from Maximum Brake Torque spark timing. In one embodiment, spark timing is changed to meet demanded torque when operating in the homogeneous mode and changing fuel flow to such cylinder to meet torque demand when operating in the stratified mode.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.